1. Field of the Invention
The present invention relates to orthogonal frequency division multiplexing (OFDM), and more particularly, to impulsive noise suppression scheme in orthogonal frequency division multiplexing.
2. Description of the Prior Art
Orthogonal Frequency Division Multiplexing (OFDM) is a multi-carrier modulation technique that can manage high degree of multi-path distortions. This technique has been used in digital audio broadcasting and has been chosen for European digital terrestrial video broadcasting.
The longer OFDM symbol duration provides an advantage because impulsive noise energy is spread among simultaneously transmitted OFDM sub-carriers. However, it has been recognized that this advantage will turn into a disadvantage if the impulsive noise energy exceeds certain threshold. Hence, Sergey V. Zhidkov proposed an algorithm for impulsive noise suppression in OFDM receivers in the paper, “Impulsive Noise Suppression in OFDM Based Communication Systems”, IEEE Transactions on Consumer Electronics, Vol. 49, No. 4, November 2003.
Please refer to FIG. 1, which is a block diagram showing an impulsive noise suppression scheme 100 in OFDM proposed by Zhidkov in the above mentioned paper. In this scheme 100, the received signal R after fast Fourier transform 110 can be expressed asRk=HkSk+Wk+Uk, k=0, 1, . . . , N−1   (Equation 1)where H is the discrete Fourier transform (DFT) of channel impulse response, S is the DFT of transmitted signal, W is the DFT of AWGN (Additive White Gaussian Noise) term, and U represents the DFT of impulsive noise, respectively. By assuming ideal channel estimation Ĥk≡Hk, the received signal after frequency domain equalization 120 can be expressed as
                                          R            k                          (              eq              )                                =                                                    R                k                                                              H                  ^                                k                                      =                                          S                k                            +                                                W                  k                                                                      H                    ^                                    k                                            +                                                U                  k                                                                      H                    ^                                    k                                                                    ,                  k          =          0                ,        1        ,        …        ⁢                                  ,                  N          -          1                                    (                  Equation          ⁢                                          ⁢          2                )            The preliminary estimation of transmitted base-band symbol, Ŝk, k=0,1, . . . , N−1, is derived from the equalizer 120 output via the “de-mapping and pilot insertion” procedure 130 by setting silent sub-carriers to zero, replacing pilot sub-carriers by known values, and de-mapping data transmission sub-carriers to nearest positions in constellation plot.
Thereafter the estimation of total noise term, Dk=Wk+Uk, is performed according to the following equation:
                                                        D              ^                        k                    =                                                    H                ^                            k                        ⁡                          (                                                R                  k                                      (                    eq                    )                                                  -                                                      S                    ^                                    k                                            )                                      ,                  k          =          0                ,        1        ,        …        ⁢                                  ,                  N          -          1                                    (                  Equation          ⁢                                          ⁢          3                )            where the total noise term D is a frequency domain representation of impulsive noise corrupted by AWGN and can be calculated by the adder 132 and the multiplier 134.
In order to reconstruct impulsive noise Ûk, the output vector {circumflex over (D)}K of the multiplier 134 is transformed into time domain {circumflex over (d)}k by means of IFFT 140. The variance of {circumflex over (d)}k could be estimated by the following equation:
                              σ          2                =                              1            N                    ⁢                                    ∑                              k                =                0                                            N                -                1                                      ⁢                                                                                                d                    k                                    ^                                                            2                                                          (                  Equation          ⁢                                          ⁢          4                )            After that, the time domain representation of impulsive noise ûk could be re-constructed by the following equation:
                                          u            ^                    k                =                  {                                                                                                                                        d                        ^                                            k                                        ,                                                                  if                        ⁢                                                                                                  ⁢                                                                                                                                                                        d                                ^                                                            k                                                                                                            2                                                                    >                                              C                        ⁢                                                                              σ                            ^                                                    2                                                                                                                                                                                                            0                      ,                      otherwise                                        ⁢                                                                                                                                      ,                          k              =              0                        ,            1            ,            …            ⁢                                                  ,                          N              -              1                                                          (                  Equation          ⁢                                          ⁢          5                )            where C is a threshold value that corresponds to small probability of false detection. Next, the frequency domain representation of impulsive noise Ûk could be transformed from the time domain representation of impulsive noise ûk by means of FFT 160.
At last, the noise-suppressed signal Rk(comp) could be calculated by an inverting mean 170, multiplier 162, and adder 164 according to the following equation:
                                          R            k                          (              comp              )                                =                                    R              k                              (                eq                )                                      -                                                            U                  ^                                k                                                              H                  ^                                k                                                    ,                  k          =          0                ,        1        ,        …        ⁢                                  ,                  N          -          1                                    (                  Equation          ⁢                                          ⁢          6                )            The computed received signal could be sent to a Viterbi Decoder 180 for further processing.
However, in this proposed scheme 100, the computation of impulsive noise Ûk involves an inverse FFT (IFFT) operation, a peak detection operation (the Peak detector 150), and a FFT operation. These operations require a substantial amount of power. Nevertheless, the computation of Ûk is necessary given the occasional existence of the impulsive noise power. Therefore, there is a need for a better scheme to omit the computation of impulsive noise when it is unnecessary.